The present invention relates to a pattern forming method and a projection exposure tool used for the method, and particularly to a pattern forming method capable of transferring various kinds of patterns much finer than the conventional ones on a specified substrate at a high accuracy, and a projection exposure tool suitable for the method.
As is well-known, lithography has been extensively used for formation of patterns of semiconductor integrated circuits or liquid crystal devices. This technique involves illuminating a reticle having a specified mask pattern, and projecting and forming an image of the mask pattern on a photosensitive film (resist film) formed on a specified substrate, thereby transferring the mask pattern on the photosensitive film. The specific examples of the above light include a ultraviolet-ray, excimer laser, electron beam and X-Ray. In this technique, a reduction type projection exposure tool has been commonly used, wherein a mask pattern formed on a reticle is transferred on a photosensitive film at a reduction rate of, for example 1/5.
As miniaturization of patterns of semiconductor integrated circuits or the like has been advanced, patterns finer than the conventional ones have been required to be formed at a resolution higher than the conventional one.
It is well-known that as the numerical aperture (NA) of a projection lens becomes larger or the wavelength of an exposure light becomes shorter, the resolution in formation of a pattern improves. However, to form a pattern of a large area on a photosensitive film, the projection lens must allow a large exposure field; consequently, the NA of the projection lens is difficult to be increased so much. The shortening of the wavelength of an exposure light, which is affected by a corresponding light source and materials of the projection lens and resist film, is nearing the critical value.
Under these circumstances where it is difficult to further increase NA and to further shorten the wavelength of an exposure light, several methods, which are capable of projecting a fine pattern smaller than the conventional resolution limit using the conventional projection exposure tool, have been proposed.
Examined Japanese Patent No. SHO 62-50811 discloses a phase shift method of generating an optical phase difference of an exposure light by a mask itself thereby significantly improving the resolution particularly with respect to a pattern having periodical apertures. Unexamined Japanese Patent No. SHO 62-67514 discloses a method, wherein even in transfer of a pattern which is substantially isolated, the resolution can be improved by further adding an auxiliary optical phase difference pattern. Unexamined Japanese Patent Nos. HEI 04-101148 and HEI 04-67515 disclose methods of improving the resolution by increasing the intensity of an illumination light only in a specified direction.
The resolution limit of a projected image is described in "Theory of Fourier Image-Formation" (Teruji Kose, issued by Kyoritsu Shuppan), p.111 and in Applied Physics, Vol. 37, No. 9 (1968), pp. 8523-859. As a technique of projecting a pattern having a high spatial frequency and being finer than the conventional resolution limit, these references propose a method of disposing grating patterns having specified frequencies on an object plane and an image plane respectively, and allowing the grating patterns to achieve functions of modulation and demodulation, respectively.
Specifically, a pattern having a spatial frequency .nu. expressed by a function of only x is disposed on a substance surface, and first grating stripes having a spatial frequency .tau. are disposed to be superimposed on the above pattern. As a result, stripes (moire rings) having the summed spatial frequency of (.nu.+.tau.) and the differential spatial frequency (.nu.-.tau.) are formed. By superimposing suitable first grating stripes on a mask pattern having a frequency .nu. larger than a coherent cut-off frequency (.lambda./NA) of a projection lens, the differential frequency (.nu.-.tau.) can be set to be not more than the cut-off frequency. The moire rings pass through an optical system of the projection lens; accordingly, by superimposing second grating stripes having the spatial frequency .tau. on the moire rings on the image surface, the mask pattern can be projected.
In the methods disclosed in the above-described references, Examined Japanese Patent No. SHO 62-50811 and Unexamined Japanese Patent No. SHO 62-67514, transparent phase shifters capable of changing the phase of an exposure light by 180.degree. are provided on alternate apertures in the opaque region of the mask, whereby the resolution is improved. In the methods disclosed in the above-described references, Unexamined Japanese Patent Nos. HEI 04-101148 and HEI 04-267515, the resolution superior to the conventional one can be obtained by adopting the so-called off-axis illumination for obliquely illuminating the mask.
In general, the resolution of a projection lens of a projection exposure tool is determined by both a wavelength .lambda. of an exposure light and a numerical aperture NA of the projection lens. The dimension R of the resolution limit is expressed by the following equation: EQU R=k.sub.1 .lambda./NA
where .lambda. is a wavelength of an exposure light; and k.sub.1 is a value depending on the exposure and development of the pattern (practically, about 0.6 to 0.8). The contrast of a projected image approaches 1 as the size of the pattern becomes larger; and it becomes 0 as the pattern size becomes smaller, that is, the spatial frequency becomes higher. In the meanwhile, a fine pattern having a spatial frequency higher than 2NA/.lambda., can not be projected. On the basis of these restrictions, the methods disclosed in the above-described references, Examined Japanese Patent No. SHO 62-50811 and Unexamined Japanese Patent No. SHO 62-67514 are intended to enhance the contrast of a projected image in a high spatial frequency region.
The methods disclosed in the above-described references, Unexamined Japanese Patent Nos. HEI 04-101148 and HEI 04-267515 are intended to slightly lower the contrast of a projected image in a low spatial frequency region and at the same time to enhance the contrast of a projected image in a high spatial frequency region. These methods are satisfactory to enhance the resolution as compared with the conventional method in the high spatial frequency region; however, they fail to project a fine pattern having a spatial frequency higher than 2NA/.lambda., because the limit of the spatial frequency of a transferable pattern is 2NA/.lambda. as described above.
On the other hand, in the method disclosed in the above-described reference, "Theory of Fourier Image-Formation" (Teruji Kose, issued by Kyoritsu Shuppan), p.111 or Applied Physics, Vol. 37, No. 9 (1968), pp. 853-859, the grating patterns having specified frequencies are respectively disposed on the object plane and the image plane, and allowed to achieve the functions of modulation and demodulation, thereby exhibiting a possibility of projecting a fine pattern having a spatial frequency higher than 2NA/.lambda.. This method, however, has the following disadvantage: namely, the second grating stripes must be disposed on the surface of the wafer and be scanned for demodulating the information of the differential frequency transmitted through the projection lens and reproducing the image of the mask pattern on the wafer; but in most cases, the actual surface of the wafer has stepped portions of about 1 .mu.m or more, and it does not allow the grating stripes for demodulation to be closely formed on the image plane having such irregularities and to be scanned.
As is apparent from the above description, as the dimension of a pattern to be formed is finer, the spatial frequency thereof becomes higher and thereby the contrast is significantly reduced. As a result, in the case of using i-line or excimer laser as an exposure light, the resolution limit becomes the order of the wavelength thereof, that is, it is difficult to form a fine pattern having a resolution limit finer than about 0.35 .mu.m or 0.25 .mu.m. Even in the above-described method of providing the optical phase difference of an exposure light to the mask itself, the practical value of k.sub.1 is about 0.35, and accordingly, it is difficult to form a fine pattern having a resolution finer than about 1/2 of the wavelength.